In optical transmitters, in particular, laser diodes, the center wavelength of the transmission spectrum is influenced by different parameters, e.g., the temperature of the transmission element which, in turn, depends on the average current intensity and the modulation frequency. A change or shift of the center wavelength is, however, particularly disadvantageous in signal transmission via a single optical waveguide in the wavelength division multiplex mode because this may respectively create or intensify the crosstalk between one channel and one or more adjacent channels.
Consequently, it is usually required to carry out a temperature adjustment of the optical transmission element in order to maintain the transmission power as well as the center wavelength of the transmission element constant within desirable limits. For example, a Peltier cooling element may be used for this purpose, with this cooling element simultaneously preventing the overheating of the transmission element at higher transmission powers. However, since the temperature of the transmission element is only measured at a certain location during temperature control, the process of maintaining constant the center wavelength of the transmission spectrum cannot be ensured with the accuracy required for transmission in the wavelength division multiplex mode. In addition, an option for tuning the transmission wavelengths usually needs to be provided in wavelength division multiplex transmission units so as to allow compensation of manufacturing-related tolerances in the transmission spectrum of the transmission elements. This requires a monitoring of the center wavelength of the transmission elements.
The option of utilizing monitor diodes that are preferably integrated in the transmission element for monitoring the (average) transmission power as known in the state of the art represents a high expenditure for wavelength division multiplex transmission units because each transmission element needs to be equipped with a monitor diode. This complicates the design of the transmission elements.